This invention relates to carbonate fuel cells and, in particular, to matrices, and methods of making same, for use in such cells.
Carbonate fuel cells offer a highly efficient and environmentally clean option for power generation. A key component in carbonate fuel cells is a porous (.about.40-50 vol. % porosity), thin (.about.0.5-1 mm) ceramic matrix filed with a carbonate electrolyte (for example, Li/K carbonate eutectic). The functions performed by the ceramic/electrolyte matrix in the carbonate fuel cell include: (1) electronic isolation of the anode and cathode compartments, (2) ionic conduction between the anode and cathode electrodes, and (3) separation of the anode and cathode compartment gases.
The ceramic materials of the matrix provide the electronic isolation, while the carbonate electrolyte present within the pores of the ceramic matrix supports ionic conduction. The matrix pore sizes are required to be in the submicron range to achieve desired capillary force for retaining electrolyte and differential bubble pressure for preventing the anode and cathode compartment gases from crossing over. Large-size pores are avoided to adequately maintain the gas separation.
During the last decade, gamma lithium aluminate (.gamma.-LiAlO.sub.2) has been mainly used as the ceramic matrix support material in the carbonate fuel cell due to its chemical stability in the molten carbonate environment. Matrices made from ultra-fine .gamma.-LiAlO.sub.2 powder (.about.0.1-0.24 .mu.m) using a tape casting technique (see, e.g., German Patent DE 3235240; U.S. Pat. No. 4,353,958; and U.S. Pat. No. 4,478,776) are able to provide a desired pore structure for retaining electrolyte and for realizing gas separation. The tape casting process involves dispersing the ceramic powder in a solvent, to which organic binders and a plasticizer are added to form a castable ceramic slurry. The slurry is subsequently casted into thin-sheet layers in a chambered tape casting machine. In this process, the solvents are evaporated by applying air and/or heating.
Organic solvents are primarily used as the tape casting vehicle in the ceramic matrix formation. However, the use of organic solvents involves a number of disadvantages. Firstly, working with organic solvents requires that appropriate protective measures be taken in order to avoid health hazards and the risk of explosion. Moreover, the organic solvents must be prevented from escaping into the atmosphere. As a result, the equipment requirements are severe. This drives up equipment costs.
A second disadvantage of using organic solvents in the tape casting process is that the ceramic slurry tends to form a surface skin due to the very fast evaporation of the organic solvents. Such skinning significantly retards the as-cast matrix tape drying rate and may trap any unreleased air bubbles in the tape. Bubbles in the matrix tape result in undesirably large pores in the matrix. These large pores can result in detrimental reactant gas crossover, thereby lowering cell performance and shortening cell life.
Despite the aforesaid disadvantages of organic solvents, they offer a more practical solvent than aqueous solvents because of the hydroscopic nature of the .gamma.-LiAlO.sub.2. Water can hydrolyze the .gamma.-LiAlO.sub.2 into lithium hydroxide and aluminum hydroxide. Although lithium aluminate may form again upon drying, the formed phase is the alpha or beta lithium aluminate phase, which phases have not been used to any great degree in matrix formation.
Besides the phase transformation, the formation of widespread aggregates/agglomerates due to the hydrolysis of .gamma.-LiAlO.sub.2 is also an obstacle to using an aqueous or water solvent in tape casting of .gamma.-LiAlO.sub.2 matrices. These widespread aggregates caused by the hydrogen-bonding of the hydrolyzed particles are very difficult to deaggregate when using only the mechanical forces produced by standard milling. As a result, a slurry with excessively high viscosity is created so as to prevent practical ceramic processing.
While few practical efforts have been made to use water as a solvent in the casting of .gamma.-LiAlO.sub.2 matrices, a tape casting method which uses such an aqueous solvent is described in a recently issued U.S. Pat. No. 5,432,138. In this method, a hydroxyl group polymer, such as polyvinyl alcohol (PVA), is used to bond the water molecules so as to prevent the hydrolysis of the .gamma.-LiAlO.sub.2. Using this method, a flexible tape can be made from a slurry of gamma lithium powder directly stirred in a PVA/water mixture.
Although this method appears to prevent the hydrolysis of .gamma.-LiAlO.sub.2 in the aqueous base processing, several concerns exist as to use of a matrix made with this method in carbonate fuel cell applications. A major concern is that such a matrix may not have a pore size distribution (which plays a key role in electrolyte storage and gas separation) adequate for carbonate fuel cells. It is believed that if the process of the '138 patent is followed, the high surface area .gamma.-LiAlO.sub.2 tends to agglomerate easily so as to form uncontrolled lumps and clusters. These agglomerates or clusters will contain nearly random-sized pores, and the largest of these pores can seriously affect the sealing efficiency of the final matrix.
Alpha lithium aluminate (.alpha.-LiAlO.sub.2) has been known to be more stable than .gamma.-LiAlO.sub.2 in the molten carbonate environment, and efforts toward using this material in forming a carbonate fuel cell matrix have recently increased. Tape casting with an organic solvent is also the technique proposed for making .alpha.-LiAlO.sub.2 matrices with a desirable pore size distribution. However, tape casting of such a material using an aqueous or water solvent has not as yet been carried out.
A difficulty in using a water solvent in the tape casting of .alpha.-LiAlO.sub.2 is the occurrence of hydration during the deflocculation step. Although the hydration product is eventually converted back into .alpha.-LiAlO.sub.2 upon drying, the tendency of high viscosity or gelation due to the hydrogen bond (H-bond) retards the deflocculation process. The extent of the gelation is strongly affected by the powder surface area, solid loading level, deflocculation time, and the dispersant used. The more the powder, i.e., the higher the solids loading, the higher the viscosity. Also, the higher the total powder surface area, the easier the gelation. Experimental data indicates that if a high surface area .alpha.-LiAlO.sub.2 (.gtoreq.20 m.sup.2 /g) is stirred long enough (.gtoreq.0.5 hour) in water, a gel or "cake" will form.
It is therefore an object of the present invention to provide a method for making a ceramic matrix for a carbonate fuel cell which overcomes the above-mentioned disadvantages of prior methods.
It is a further object of the present invention to provide a method for making a ceramic matrix having a desired pore size distribution and in which .alpha.-LiAlO.sub.2 and an aqueous solvent are used.